The sun freely emits ultraviolet (UV), visible and infrared (IR) radiation, much of which is absorbed by the atmosphere. Solar radiation that is transmitted through the atmosphere and reaches the earth's surface includes UV-A radiation (320-400 nm), UV-B radiation (290-320 nm), visible light (400-700 nm) and near IR radiation (700-1400 nm). The ocular lens of humans in its normal, healthy state freely transmits near IR and most of the visible spectrum to the retina, but the lens acts to absorb UV radiation to avoid damage to the retina. The ability to absorb near UV and the violet-blue portion of the visible spectrum changes throughout life. In infancy, the human lens will freely transmit near UV and visible light above 300 nm, but with further aging the action of UV radiation from the environment causes the production of yellow colorants, fluorogens, within the lens. Some studies indicate that by age 54 the lens will not transmit light below 400 nm and the transmission of light between 400 and 450 nm is greatly diminished. As the lens ages it continuously develops a yellow color, increasing its capacity to filter out near UV and violet-blue light. Therefore, after cataract removal the natural protection provided by the aged human lens is also removed. Cataracts are typically replaced by an intraocular lens (IOL). If the brain is stimulated by signals caused by the visible light that has not been transmitted for many years, discomfort can result. Development of IOL materials with an absorption similar to aged human lens material would be a welcome improvement to the art.
Although yellow colorants exist, many such colorants are unsuitable for use in artificial lens material due to their tendency to leach out of the IOL after it is inserted in the eye or during solvent extraction associated with lens manufacture. A yellow colorant that is covalently bonded to lens materials would be thus be a desirable improvement in the manufacture of artificial lens materials. Efforts have been made to develop such a lens material. One obstacle of such efforts has been finding a polymerizable compound that will produce IOLs having an absorption profile that carefully matches that of the aged human lens, especially in the visible spectrum. If the IOL absorbs more than the lens in portions of the visible spectrum, visible acuity can be diminished. If the IOL absorbs less in the visible spectrum, the discomfort discussed above can result. Another obstacle that such efforts have faced has been the need to use a combination of multiple compounds to achieve a careful match with the human lens. Use of multiple compounds can result in a more complicated manufacturing process, along with increased production and materials costs. A polymerizable yellow colorant that matches the absorption spectra of the human lens and reduces the need for multiple colorants in an IOL would be a welcome improvement in the art.
More broadly, the development of yellow colorants and absorbers of ultraviolet light that can be covalently bonded to various polymeric materials would have numerous other uses beyond that in artificial lenses. For example, such colorants could be used with a wide array of polymeric applications in which the appropriate absorption spectrum is desired. Thus, what is needed in the art is novel yellow colorants and ultraviolet light absorbers (UVAs) that are more economical, and have spectral properties that better suit their target applications.